ENDOTHELIAL CALCIUM SIGNALLING REGULATES CAPILLARY BLOOD FLOW DYNAMICS <EM>IN VIVO</EM>

Endothelial calcium (Ca²⁺) signalling is a regulator of vascular tone, yet its in vivo role in controlling cerebral microvascular function remains incompletely understood, in part because of limitations in imaging and manipulating (Ca²⁺) specifically within endothelium. Using in vivo two-photon microscopy in transgenic mice expressing GCaMP8m selectively in cerebral endothelial cells, we first examined the spatial and temporal kinetics of Ca²⁺ transients. Under resting conditions, endothelial cells exhibit sparse, low frequency transients of variable durations (1-30s). These Ca²⁺ transients showed a tendency to precede vessel dilation, but could also follow as well. To determine what role endothelial Ca²⁺ transient play in regulating cerebral blood flow (CBF), we triggered endothelial Ca²⁺ waves with injection of deschloroclozapine (DCZ) in mice expressing DREADD hM3Dq receptors in endothelial cells. Within 20-30s after injection of DCZ, we observed a significant elevation of Ca²⁺ in endothelial cells that preceded a 5-20% increase in vessel diameter, primarily in the penetrating arteriole and associated 1^st-3^rd order capillary branches. Laser doppler experiments confirmed the significant increase in CBF associated with stimulating endothelial Ca²⁺ in DREADD expressing mice. Experiments are underway to determine if dampening endothelial Ca²⁺ transients affect the cerebral microcirculation under resting and stimulation conditions. These findings highlight a key role for endothelial Ca²⁺ signalling in the regulation of cerebral blood flow and microvascular dynamics in vivo.